Temperature sensors are well known in the integrated circuit art. Typically, a temperature sensor provides an output voltage whose magnitude equates to the temperature that the circuit senses.
One temperature sensor 100 is shown in FIG. 1, and is taken from U.S. Pat. No. 6,867,470 as a good illustration of the problems indicative of prior art temperature sensors. As shown, temperature sensor 100 includes a current mirror 130 comprised of P-channel transistors 135a-e in (in this case) five output stages 125b, each of which passes the input of I. This current from each stage is met by five PNP (bipolar) transistors 137a-e, which comprise in effect 5 P-N junctions in series. The base-to-emitter voltage of these P-N junctions, Vbe(a)-(e), is a function of temperature, and essentially such voltage changes by about −2 mV per every degree Celsius. Aside from this temperature dependence, the Vbe for each junction is on the order of about 0.6 Volts at room temperature (25 degrees Celsius). Accordingly, the output voltage, Vout is on the order of 3.0V (0.6V*the five stages), and its sensitivity is on the order of about −10 mV/C (−2 mV*5).
More stages could be used to increase the temperature sensor 100's sensitivity, but this comes at a price. While each junction added to the circuit adds sensitivity (i.e., another −2 mV/C worth at the output), it also adds another 0.6V drop. Accordingly, as more and more junctions are used, the power supply voltage, Vdd, must be increased accordingly. For example, for the temperature sensor 100 of FIG. 1 to function as desired over an appropriate temperature range (e.g., −50 to 100 degrees C.), the power supply voltage must be at least 3.5V (i.e., about 3.0V for the P-N junctions and another 0.5V for proper Vds voltage drops across the current mirror transistors 135). But this is an unfortunate limitation, especially when considering that many modem-day integrated circuits have power supply voltages that are lower than 3.5V. This minimum power supply limitation can be alleviated by removing some of the stages/junctions from the circuitry 100, but this comes at the price of reduced sensitivity. In other words, temperature sensor circuits of the prior art tend to offer either high sensitivities, or flexible power supply operating values, but not both as would be desirable.
It is therefore a goal of this disclosure to provide embodiments of temperature sensors that are both highly sensitive over extended temperature ranges and capable of working at wider power supply ranges and in particular at low power supply values.